Indian southwest monsoon: onset, branches and the basic mechanism
The southwest monsoon is India’s main rainy season, generally spanning June to September; its Arabian Sea and Bay of Bengal branches bring rain to different regions.
The southwest monsoon is India’s principal rainy season, generally associated with June–September. The flow divides into Arabian Sea and Bay of Bengal branches, whose paths, topography and moisture supply help determine regional rainfall patterns.
Why the monsoon develops
Seasonal heating creates pressure contrasts between the Asian landmass and surrounding ocean, while the seasonal shift of large-scale circulation helps draw moisture-bearing winds toward the subcontinent. The monsoon is a complex circulation system, not simply a cloud moving north.
The Western Ghats, Himalaya and other terrain features influence where air rises and where rain falls. That is why nearby regions can receive very different rainfall amounts even during the same monsoon phase.
What onset and withdrawal mean
The India Meteorological Department tracks onset and withdrawal using defined meteorological criteria. A date announced for one state or location should not be treated as the onset date for all of India.
For exam answers, distinguish the broad season from local rainfall, and use IMD bulletins for a current-year onset or withdrawal date.
Editorial note
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Frequently asked questions
Which two broad branches of the southwest monsoon are commonly taught in Indian geography?
The Arabian Sea branch and the Bay of Bengal branch.